Automatic noise suppression circuits



Sept. 6, 1938. w. VAN B, ROBERTS 4 AUTOMATIC NoIsE SUPPRESSION CIRCUITS Filed June 27, 1956 vvvvvvvv uuml nnn 11111111 11111111.

Patented Sept. 6', 1938 UNITED STATES PATENT OFFICE Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 27,

9 Claims.

My present invention relates to automatic gain control circuits, and more particularly to automatic control circuits for suppressing background noise impulses in signal transmission systems.

Various types of automatic background noise suppression circuits have been disclosed in my application Serial No. 326,990, filed December 19, 1928, and in these circuits the transmission efficiency of the audio frequency amplifier network is automatically regulated in such a manner that the transmission eiiiciency is substantially impaired when the received signal amplitude de- .creases below a desired intensity Value. Briefly, this is accomplished by maintaining at least one of. the audio amplifiers at cut-E bias for signals below the desired amplitude, and utilizing a direct .current voltage, dependent upon the received :signal amplitude, for decreasing the cut-off bias lwhen the signal amplitude increases above the desired amplitude.

In such types of noise suppression networks, if the automatic volume control circuit does not act to maintain substantially constant amplitude at `the demodulator input over the entire range of useful signal strength, then there is a tendency for the cut-off bias on the audio amplifier to be reduced more in the presence of strong signals than in the case of the weak signals. Since the negative cut-off bias should only be reduced, when receiving desired signals, to an extentY such that a normal operating negative bias remains for the audio amplifier, it will be seen that the compensating positive voltage may reduce the negative bias on the audio amplifier to an extent such that the audio amplifier grid will become positive and draw grid current, thereby introducing distortion.

Accordingly, it may be stated that it is one of. the main objects of my present invention to provide a device, in an automatic background noise suppressor network of the type employing a positive voltage to overcome the effect of a noise cut- `off audio amplifier bias, which functions to automatically reduce the magnitude of the positive voltage when the latter increases above a predetermined value.

Another important object of the invention is to providev in a radio receiver equipped with automaticvolume control, a means for rendering the audio system inoperative by a cut-off biasing voltage whenever signals fall below a predetery mined strength thereby to avoid the rather noisy condition that occurs when the amplification automatically becomes very high in the absence of sufficient signal strength to give a useful audio Y output; an additional voltage being supplied by a 1936, serial No. 87,608

(Cl. Z50-20) diode rectifier for reducing the aforesaid biasing voltage to a value suitable for normal amplification in the presence of signals of suitable intensity; and an auxiliary means being provided in electrical association with said diode rectifier for maintaining the effective bias of the audio amplifier at substantially a constant and suitable value over a wide range of signal intensity.

Another object of the invention is to provide a radio receiver of the type including a radio frequency amplifier and an audio frequency amplifier, a signal diode rectifier being provided to function simultaneously as a demodulator; a source of automatic volume control (AVC) voltage for the radio frequency amplifier; and a source of direct current voltage for rendering a normally inefficient audio amplifier fully operative in the presence of receivable signals.

Still another object of the invention may be stated to reside in the combination with a demodulator and audio frequency amplifier, of manually adjustable means for regulating the intensity of audio voltage fed to the audio ampliiier and the type of audio amplification best suited for the audio voltage intensity; an automatic background noise suppression network being additionally provided for regulating the audio transmission efficiency in dependence on the received signal amplitude. v

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organi- Zations whereby my invention may be carried into effect.

In the drawing:

Fig. l shows an embodiment of the invention wherein the quieting action is removed by a network entirely separate from the demodulator and AVC arrangement,

Fig. 2 shows a modification wherein a common diode is employed for producing AVC voltage; as a detector; and` for producing the voltage required to bringthe audio tube bias to the normal operating value.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, Fig. 1 shows that portion of a superheterodyne receiver which is necessary to an understanding of the invention.v The numeral l designates a resonant circuit tuned to the operating IF, and th-e circuit 'I may be disposed in the plate circuit of a preceding IF amplifier; or it may be arranged in the plate circuit of the first detector tube. Those skilled in the art Will readily understand that the networks preceding circuit I may comprise the usual signal collector feeding into one or more stages of tunable radio frequency amplification. The first detector is also tunable, and is fed with locally produced oscillations from a local oscillator which is simultaneously tunable over the desired signal frequency range with the radio frequency amplifiers and the first detector; The

signal range may be in the broadcast band of 500.

to 1500 k. c.; and the operating IF may be chosen from a value of 75 to 465 k. c.

The circuit I is coupled to the IF tuned circuit 2, and the latter feeds a pair of IF ampliersr3 and 4. The IF amplifier 3 is Coupled through the' transformer M to the second detector, or demodulator diodej, ,The primary and secondary circuits of transformer M are each tuned to the operating `IFand the load resistor '.6 is arranged in series between the grounded cathode of diode 5 and the ,low alternatingV potential side of the input circuit .'I of diode 5.

The direct current voltage component of vthe rectified IF currents, owing through resistor B, is used for AVC of the preceding signal transmission tubes. Thus, the numeral Sjdenotes the AVC lead from the anode side of resistor 6 tothe grid circuits of the preceding tubes whose gains are to be controlled. The usual filter Aelements 9 and I0 are employed in the AVC lead signal grid of tube I2 is connected to any desired point Yon resistor 6 through a path which includes the audio coupling condenser I3 and the adjustable tap I4. The condenser I5 is arranged .in shunt'with resistor 6 for by-passing IF currents. The` cathode of ampliiier I2 is grounded, and the plate circuit thereof is coupled, through an audio transformer- M1, to the signal grids of ythe push-pullaudio amplifier stage I 6. The audio transformer M2 couples the output of the push- Vvpull stage I6 to any desired type of reproducer.

The normalgoperating bias for the push-pull stage I6 is derived from the direct current'voltage source I'I which has a resistor I8 in shunt therewith. The cathodes of the tubes of stage I6 are "grounded, and the positive side of source I'I is grounded. The signal grids of the push-pull arranged tubes are connected through a tap I9 to any desired point on resistor I8.

The numeral denotes a manual volume control device which mechanicallyrcouples adjustable taps I4 andY I9 so that a predetermined relation exists between the audio voltage transmission to amplifier I2 andth-e'negative bias Yapplied to the grids .of the push-pull stage I6.' Specifically, the manual volume .control relates the movement of taps I4 and I9 so that when tap I 4 is adjusted to provide a low level output, then the tap- I9 is adjusted to a point on resistor I8 such that theY bias on push-pull stage I6 is of a value which gives Vclass A amplification Vwith its well known freedom from distortion. On the other hand, for high level output adjustment of tap I4, the tap I9 is adjusted so that the bias on the push-pull stage is u such as tube 3, increases.

vcrease in gain of the signal transmission tubes IIF channel which includes the amplier 4.

increased to make the stage act as a class B amplifier, thus permitting it to handle greater amounts of energy.

The action of the AVC is too well known to describe it in detailed fashion. Briefly, as the signal amplitude of received signals increases, the direct-current voltage developed across resistor 6 increases, and therefore-the negative bias applied to the controlled signal transmission tubes,

This results in a dewith the result that the signal carrier amplitude at the input circuit 'I of demodulator 5 remains substantially'constant. Since the gain of each of the controlled tubes is a maximum when weak signals are received, due to the absence of AVC biasuof sufficient strength, such controlled tubes amplify noise impulses to a very great extent. A noisy condition occurs in the usual AVC receiver in the absence of sufficient signal strength to give "a useful audio output.

. Accordingly, a background noise suppression Asystem is employed to overcome this noise effect. Forsignal amplitudes less than a predetermined valuethe amplifier I2 will be rendered substantially inoperative, and this is accomplished by connecting between the cathode and control grid of the tube av direct current voltage source 2|. This source has its negative terminal connected to the control grid through a path which includes resistors R1, R2 and'R3.

The-source 2I applies a negative bias to the .control grid of tube I2 suiiicient to prevent the tube from relaying audio currents impressed upon its grid through coupling condenser I3. When signals of a useful magnitude are turned in how- ;ever, it is desired that this high biasing voltage be reduced toY anextent sufficient to leave a bias on the lgrid of tube I2 enabling the tube to operate most efficiently. This is accomplished by providing la positive direct current voltage in Ikpolarity opposition to the direct current voltage from source 2I, and the positive voltage varying strength depending upon the received signal amplitude. Y This variable positive voltage is provided by the The diodef22 lhas connected between its anode and cathode a series path including resistor R1 and the tuned secondary circuit 23 of the IF transformer M3; the resistor R1 being by-passed for intermediate frequencies by condenser 24. When the signals of desired magnitude are received, thecurrent rectified by diode 22, and iiowing through'resistor R1 produces a voltage drop suiiicient to reduce the negative cut-off bias on amplifier I2 to a suitable value for efficient operation.

In other Words, as long as the signal amplitude at circuitr23 is less than the amplitude at which 'a useful audio output is secured, whatever direct currentfvoltage is developed across resistorV R1 is not suflicient'to reduce the cut-off bias from sourceli to a point permitting efficient operation of tube I2. VWhen the signal amplitude increases .tothe Vdesired level, then the drop across resistor R1 is suflicient to-reduce the `negative bias from-source 2| so vas tol leave a residual desired -voperating negative bias on the grid-of tube I2.

'If theAVC action in the receiver is sufficiently powerful so thatv overA the Yentire range of useful that the signal grid of amplifier 4 is under AVC regulation.

However, in the event that the AVC action is not very strong, there would be a tendency fory the effective bias on tube I2 to be reduced more in the presence of strong signals than in the vcase of weak signals. ItV may even occur that the positive voltage across resistor R1 counteracts the negative voltage from source 2| to such an extent that the grid of tube I2 becomes positive and causes distortion. To hold the effective bias on tube I2 more nearly constant there is. provided the diode 25 whose anode is connected at the junction of resistors R2 and R3, and whose cathode is connected to a point of source 2| having a potential equalr to the desired operating bias for tube I2. The resistor R2 is traversed by current flowing through diode 25, it having been pointed out previously that this resistor is in the direct current path between the control grid and cathode ofampliiier I2.

So long as the grid of tube I2 is more negativeY than it should be for eflicient operation, no current flows through diode 25, but if the grid of the amplier becomes less negative than it should be, current flows through diode 25 and produces across resistor R2 a voltage drop which tends to make the grid of tube I2 more negative. In other words when diode 25 becomes conductive, a voltage is created across resistor R2 which opposes the excess voltage developed across resistor R1 by the relatively stronger signal. The result is that the bias on tube I2 will be of a satisfactory value over a wide range of signal strengths, while for extremely weak signals the bias rapidly becomes so great as to prevent the relaying of noise by tube I2.

Condenser 26 is connected between the positive side of source 2I and the anode of diode 25, and functions to prevent any audio voltage from rectiiication due to diode 22, from being developed across the diode 25. The resistor R3 is provided to prevent the audio voltage impressed upon the grid of tube I2 from reaching diode 25.

The functions of diodes 22 and 5 may be combined in such a manner that a single diode performs the three functions of demodulation; AVC' source; and suppression bias removal. In Fig. 2 such an arrangement is shown; and in this figure it will be observed that the diode 5 is coupled to the output of the IF amplifier 3 to receive IF energy therefrom. The anode of diode 5 is connected to the low alternating potential side of the output circuit 3 of the amplifier 3; the couplingbeing made throughla condenser 3D. The cathode of diode 5 is connected to any desired point on the coil of circuit 3 through a path which includes the condenser 3l vand the adjustable tap 32, The tap 32 may be adjusted to a point on the coil 33 such that the impedances of the coupled circuits are satisfactorily matched.

Between the cathode and anode of diodge 5' is also connected a resistor, and a point thereon is grounded thereby providing the resistor portions 34 and 35. The audio amplifier I2 has its control grid connected to its cathode through a direct current path which includes in series the resistors 36 and 31, as well as IF choke coil 38, the resistor 34 and the negative direct current voltage source 2|'. The audio-frequency by-pass condenser 40 is connected in shunt across resistors 36 and 31, and the diode 25 is connected between the junction of resistors 36 and 3'I, and a predeterminedpoint on source 2|.

In the arrangement of Fig. 2 the intermediate frequency voltage is impressed upon the diode 5 which has across it the resistor 34-35 as a load resistance. The junctionof resistors 34 and 35 is maintained at a fixed direct current potential so that the lower end of resistor 35 develops a negative potential which is used for AVC. The cathode sideof resistor 34 develops a positive potential with a superposed audio frequency variation. This audio variation is fed to the grid of audio tube I2 through the path including the choke 38 and the large capacity 45.

The direct current component of the potential across resistor 34 is fed to the grid of tube I2 through the path including choke coil 38 and resistors 3i and 36. The cathode of tube I2 is maintained positive by the source 2I, and the latter has sufficient voltage to cut off the flow of plate current through tube I2 in the absence of suiiicient voltage developed across resistor 34 v to reduce the cut-off bias to the desired extent. The diode 25 functions in the manner described in connection with Fig. 1. It cooperates with resistor 31 to provide a compensating negative direct current voltage for the grid of tube I2 whenever the drop across resistor 34 is suiciently great to render diode 25 conductive. Resistor 35 functions in a manner similar to resistor R3 in Fig. 1, while choke coil 38 prevents IF voltage from being impressed anywhere except across diode 5.

It will therefore be seen that in Fig. 2 the diode 5 and its associated circuit performs the three functions noted heretofore, and that the additional diode 25 acts to maintain the bias on audio tube I2 substantially constant regardless of Whether relatively strong signals are received, or whether weak stations are being tuned in. Instead of employing separate diodes 5 and 25, a tube of the 6II6 type maybe employed, such a tube being provided with independent cathodes and anodes and thereby furnishing independent diode rectiers. Furthermore, in Fig. 2 there is shown a manual volume control device il which may be employed in place of the manual volume control 20 of Fig. 1. It will be observed that the control device 4I acts to vary the intensity of the audio voltage applied to the grids of the push-pull stage I6.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What is claimed is:

1. In combination with a source of signal waves and an audio frequency amplifier, a 'diode rectifier including a resistor in its space current path, means coupling the source and diode whereby said diode rectiiies impressed waves and `produces a voltage drop across the resistor which varies in accordance with signal strength, means for producing a predetermined negative bias for said audio amplifier, means for establishing the negative end of said resistor at a fixed potential, and means connecting the positive end of said resistor to the input electrodes of said audio amplifier whereby the varying positive potential at the positive end of the resistor is utilized in opposition to said negative bias ofthe audio ampliiier and additional means, responsive to an increase in said positive potential above a desired value, for biasing said amplifier in a negative sense.

'lil

2. In.; combination with an audio'amplifier'in-Y cluding means for maintainingthe amplifier nor-v mally cut off, a source of signal waves, a dioderectifier coupled to said source and including a resistor in circuit therewith whereby rectified signal currents flow through the resistor, means for establishing a point of said resistor at a fixed potential, and means for connecting the input grid of said audio amplifier to a point on said resistor which is positive with respect to the fixed potential point, whereby said cut-off bias is opposed by the positive potential of said positive point, and additional means, responsive to an increase in said positive potential above a desired value, for bias-V ing said amplifier in a negative sense.

`3. In a system as defined in claim 2, additional.

means for impressing the audio frequency component of said rectified currents upon the input grid of said audio amplifier.

4. In a superheterodyne receiver of the type including a. source of intermediate frequency energy, a diode rectifier and an audio amplifier, a fixed source of bias for said audio amplifier suiiicient to cut ofiV its `action entirely, means in circuit with said diode for developing a .voltage to reduce the effective bias on said audio amplifier to a suitable value for efficient operation, and additional means in electrical circuit with said rectifier and audio amplifier input electrodes and responsive to an excessive increase in said reducing voltage for preventing said effective bias from being reduced appreciably below said suitable value in the presence of strong signals.

5. In a superheterodyne receiver having a negatively biased audio frequency amplifier tube, a unidirectional conductor connected between a point having the same potential as the grid of said tube and a point having a'potential whose value is a suitable grid potential for efficient operation of said tube, said unidirectional conductor being so poled as to permit passage of current only when the negative potential ,difference be tween the amplifier grid and cathode is less than said suitable value, and a resistance arranged to be traversed by said current to produce a voltage drop in said grid circuit tending to prevent the effective negative bias from falling appreciably below said suitable value. v

6. In combination with a source of signals an an audio amplifier, a diode rectifier having a resistance load circuit, an intermediate point of said load being maintained'at a constant potentialrso VVthat the relative potential becomes positive at one gend, and the other end becomes negative when :signals are impressed from the source upon the diode circuit, said amplifier having its cathode connected to said intermediate point, an automatic volume control circuit connected to the said negative potential end, means for applying a bias to the audio amplifier which is sufficiently high to render the same inefiicient,-and means for applying the potential at the positive end of said load to the audio amplifier to neutralize the aforesaid end, and the other end becomes negative whenY signals areimpressed from the source upon the diode circuit, an automatic volume control circuit connected to the said negative potential end, and means for impressing the audio frequency voltage at the positive end of said load upon said audio frequency amplifier, means for applying a bias to the audio amplifier which is sufficiently high'to render the same inefficient, means for applying the direct current potential at therpositive end of said load to the audio amplifier to neutralize said bias, and additional means for preventing said bias frornbeing reduced appreciably below a predetermined operating value in the presence of relatively strong signals.

8. In combination with an audio amplifier including means for maintaining the amplifier normally cut 01T, a source of signal waves, a diode rectifier coupled to said source and including a resistor in circuit therewith whereby rectified signal currents flow through the resistor, means for establishing a point of said resistor at a fixed potential, means for connecting the input grid of said audio amplifier to a point on said resistor which is positive with respect to theA fixed potential point, whereby said cut-off bias isopposed by the positive potential of said positive. point, a second diode, means for normallyrendering the second diode non-conductive, a second resistor in series with said first resistor in the space current path of' the second diode, whereby the second diode becomes conductive when the direct current voltage drop across said first resistor exceeds the cut-off bias on the second. diode, and means for impressing the negative biasproduced acrossthe second resistor on the signal-grid of said audio amplifier.

9. In combination with a signalftransmission tube whose input electrodesareA coupled to a source of signals, means establishing the gain of said tube at Van abnormally lowv Yvalue in the absence of signals above a desired intensity, means responsive to an increase in signal intensity above the desired intensity for opposing the effect of said first means and restoring said gainl'tol a normal value, and auxiliary means, responsive to the action of said opposing means, for-reducing the tube gain when said opposing Vmeans increases said gain abnormally.

WAL'I'ERY VAN B` ROBERTS. 

